The invention relates to a tension leg platform, and to a method of installing such a platform.
To provide stable platforms for the exploitation of subsea oil and gas reservoirs in deep water offshore, it is known to use fixed multi leg support structures. These support structures have been fixed in position by piles driven into the seabed, and typically have had drilling and production facilities in a deck assembly resting on top of the legs. The deck assembly has been supported on the piles by the legs acting in compression.
In increased water depths beyond say 1200 ft or 365 m (eg. 530 m for the Jolliet field in the Gulf of Mexico) fixed support structures based on piles with legs acting in compression are impractical. To provide a stable platform in these circumstances, the concept of a "Tension Leg Platform" has been developed. A tension leg platform for the Jolliet field was featured in OTC Papers 6359/60 presented at the 22nd OTC at Houston, Tex. in 1990.
The tension leg platform has a buoyant hull portion, held down against its own buoyancy by tethers arranged around its periphery. These tethers extend vertically downward to the seabed, where they are secured by piled or weighted anchor assemblies.
Known tension leg platform concepts have had groups of tethers arranged at each of three, four or six peripheral housings, and typically there may be three tethers at each of four corners. The complexity of such an arrangement, and the difficulties in devising economical inspection and maintenance procedures have led to a requirement for an alternative arrangement.
One particular tension leg platform concept, developed for a water depth of 4000 ft or 1220 m in the Gulf of Mexico, has been described as a single-leg tension-leg platform (STLP). This concept is outlined on pg 17 of the publication "Offshore" for July 1988. Each corner column has two spring buoy anchor lines to restrict lateral movements, and the tension leg beneath a central column is designed to resist heave motions alone. While this concept is referred to as a `single leg` platform, it is clear that in fact the `single leg` comprises six welded-body pipe tendons designed to resist tension and hydro static pressure.
Another tension leg platform concept is illustrated in FIG. 1A of UK Patent Specification 2250767A. This shows a monopod compliant platform in the form of a single column Tension Leg Well Jacket (TLWJ) with one or more tendons. In the single tendon embodiment, drilling and subsequently production risers (deployed from a separate semisubmersible vessel) are disposed around the outside of the single tendon. This configuration leaves the risers exposed to the hazards of ship impact.